Methods of forming circuit-protection devices

Abstract

Methods of forming a circuit-protection device include forming a dielectric having a first thickness and a second thickness greater than the first thickness over a semiconductor, forming a conductor over the dielectric, and patterning the conductor to retain a portion of the conductor over a portion of the dielectric having the second thickness, and to retain substantially no portion of the conductor over a portion of the dielectric having the first thickness, wherein the retained portion of the conductor defines a control gate of a field-effect transistor of the circuit-protection device.

What is claimed is: 
     
       1. A method of forming a circuit-protection device, comprising:
 forming a dielectric having a first thickness, having a second thickness greater than the first thickness, and having a third thickness greater than the second thickness, over a semiconductor; 
 forming a conductor over the dielectric; and 
 patterning the conductor to retain a first portion of the conductor over a first portion of the dielectric having the second thickness and defining a control gate of a first field-effect transistor of the circuit-protection device, to further retain a second portion of the conductor over a second portion of the dielectric having the second thickness and defining a control gate of a second field-effect transistor of the circuit-protection device, to further retain a third portion of the conductor over a first portion of the dielectric having the third thickness and defining a control gate of a third field-effect transistor of the circuit-protection device connected in series with the first field-effect transistor, and to further retain a fourth portion of the conductor over a second portion of the dielectric having the third thickness and defining a control gate of a fourth field-effect transistor of the circuit-protection device connected in series with the second field-effect transistor, and to retain substantially no portion of the conductor over a portion of the dielectric having the first thickness; 
 forming a first contact to a source/drain of the third field-effect transistor adjacent a first isolation region; and 
 forming a second contact to a source/drain of the fourth field-effect transistor adjacent a second isolation region; 
 wherein a distance between the first contact and the first isolation region is greater than a distance between the second contact and the second isolation region. 
 
     
     
       2. The method of  claim 1 , wherein patterning the conductor to retain substantially no portion of the conductor over the portion of the dielectric having the first thickness comprises patterning the conductor to retain substantially no portion of the conductor over any portion of the dielectric having the first thickness. 
     
     
       3. The method of  claim 1 , wherein forming the dielectric comprises forming portions of the dielectric at different stages of forming the circuit-protection device. 
     
     
       4. The method of  claim 1 , wherein forming the dielectric comprises forming the portion of the dielectric having the first thickness after forming at least some dielectric over a portion of the semiconductor corresponding to the first portion of the dielectric having the second thickness. 
     
     
       5. The method of  claim 1 , wherein forming the dielectric comprises forming the portion of the dielectric having the first thickness after forming at least some dielectric over a portion of the semiconductor corresponding to the first portion of the dielectric having the third thickness. 
     
     
       6. The method of  claim 1 , wherein forming the dielectric comprises forming the first thickness of the dielectric concurrently with forming the second thickness of the dielectric and with forming the third thickness of the dielectric. 
     
     
       7. The method of  claim 1 , further comprising removing the portion of the dielectric having the first thickness. 
     
     
       8. The method of  claim 1 , wherein forming the dielectric having the first thickness and having the second thickness greater than the first thickness comprises forming the dielectric to have the first thickness less than 0.8 times the second thickness. 
     
     
       9. The method of  claim 1 , wherein forming the dielectric having the first thickness and having the second thickness greater than the first thickness comprises forming the dielectric to have the first thickness less than 0.6 times the second thickness. 
     
     
       10. The method of  claim 1 , wherein forming the dielectric having the first thickness, having the second thickness, and having the third thickness comprises forming the dielectric such that the first thickness, the second thickness and the third thickness are contiguous. 
     
     
       11. A method of forming a circuit-protection device, comprising:
 forming a dielectric having a first thickness, having a second thickness greater than the first thickness, and having a third thickness greater than the second thickness, over a semiconductor, wherein forming the dielectric comprises:
 forming a first portion of the dielectric over the semiconductor; 
 forming a second portion of the dielectric over the semiconductor, wherein forming the second portion of the dielectric includes, and increases a thickness of, the first portion of the dielectric; and 
 forming a third dielectric over the semiconductor, wherein forming the third dielectric includes, and increases a thickness of, the second portion of the dielectric; 
 
 forming a conductor over the dielectric; and 
 patterning the conductor to retain a first portion of the conductor over a first portion of the dielectric having the second thickness and defining a control gate of a first field-effect transistor of the circuit-protection device, to further retain a second portion of the conductor over a second portion of the dielectric having the second thickness and defining a control gate of a second field-effect transistor of the circuit-protection device, to further retain a third portion of the conductor over a first portion of the dielectric having the third thickness and defining a control gate of a third field-effect transistor of the circuit-protection device connected in series with the first field-effect transistor, and to further retain a fourth portion of the conductor over a second portion of the dielectric having the third thickness and defining a control gate of a fourth field-effect transistor of the circuit-protection device connected in series with the second field-effect transistor, and to retain substantially no portion of the conductor over a portion of the dielectric having the first thickness; 
 forming a first contact to a source/drain of the third field-effect transistor adjacent a first isolation region; and 
 forming a second contact to a source/drain of the fourth field-effect transistor adjacent a second isolation region; 
 wherein a distance between the first contact and the first isolation region is greater than a distance between the second contact and the second isolation region. 
 
     
     
       12. The method of  claim 11 , wherein forming the first portion of the dielectric over the semiconductor comprises exposing a first area of the semiconductor to a first thermal oxidation process. 
     
     
       13. The method of  claim 12 , wherein forming the second portion of the dielectric over the semiconductor comprises exposing a second area of the semiconductor to a second thermal oxidation process, and wherein the second area of the semiconductor comprises the first area of the semiconductor. 
     
     
       14. The method of  claim 13 , further comprising removing a portion of the second portion of the dielectric over a third area of the semiconductor, wherein the third area of the semiconductor is a subset of the second area of the semiconductor not containing the first area of the semiconductor. 
     
     
       15. The method of  claim 14 , wherein forming the third dielectric over the semiconductor comprises exposing the second area of the semiconductor to a third thermal oxidation process. 
     
     
       16. A method of forming a circuit-protection device, comprising:
 forming a dielectric having a first thickness, having a second thickness, and having a third thickness, over a semiconductor, wherein the third thickness is greater than the second thickness, and the second thickness is greater than the first thickness; 
 forming a conductor over the dielectric; 
 patterning the conductor to retain a first portion of the conductor over a first portion of the dielectric having the second thickness and defining a control gate of a first field-effect transistor of the circuit-protection device, to retain a second portion of the conductor over a first portion of the dielectric having the third thickness and defining a control gate of a second field-effect transistor of the circuit-protection device connected in series with the first field-effect transistor, to retain a third portion of the conductor over a second portion of the dielectric having the second thickness and defining a control gate of a third field-effect transistor of the circuit-protection device, to retain a fourth portion of the conductor over a second portion of the dielectric having the third thickness and defining a control gate of a fourth field-effect transistor of the circuit-protection device connected in series with the third field-effect transistor, and to retain substantially no portion of the conductor over a portion of the dielectric having the first thickness; 
 forming a first contact to a source/drain of the second field-effect transistor adjacent a first isolation region; and 
 forming a second contact to a source/drain of the fourth field-effect transistor adjacent a second isolation region; 
 wherein a distance between the first contact and the first isolation region is greater than two times a distance between the second contact and the second isolation region. 
 
     
     
       17. The method of  claim 16 , further comprising:
 performing a planarization process between forming the conductor and patterning the conductor. 
 
     
     
       18. The method of  claim 16 , wherein patterning the conductor comprises plasma etching the conductor. 
     
     
       19. The method of  claim 16 , wherein forming the dielectric comprises forming the dielectric to have the first thickness less than 0.6 times the second thickness. 
     
     
       20. The method of  claim 16 , wherein forming the dielectric comprises forming the dielectric to have the first thickness over a first portion of the semiconductor between a location for forming the first contact and a location for forming the first isolation region, but not over a second portion of the semiconductor between a location for forming the second contact and a location for forming the second isolation region.